Laser machining or cutting of materials relies on rapid local heating of the material. This usually results in a transition from the solid to the liquid phase and, at very high incident peak laser power, may even result in volatilization of the material. In order to obtain optimum laser cutting speed with minimal adhesion of slag or laser beam-generated debris in the vicinity of the kerf (i.e., the cut formed by the laser), an assist gas is used to transport the molten material away from the kerf. A laser cutting process can normally be accelerated by directing a stream of high velocity assist gas, e.g., air, at the laser beam impingement area of the material. In the case of cutting a material in sheet or plate form, the molten material is blown through the cut by the assist gas. This blowing action reduces the availability of the material inside the kerf for resolidification or laser energy absorption, thus accelerating the cutting process. The usual practice is to produce the assist gas stream by means of a gas nozzle having an orifice that is larger than the focused laser beam, located near the focal point of the beam, coaxial to the beam, and disposed so that the direction of the gas stream is normal to the surface of the material being cut. Various forms of gas assist laser nozzles are known. See, for example, the nozzles disclosed in U.S. Pat. No. 4,728,771 issued Mar. 1, 1988 to F. Sartorio and U.S. Pat. No. 5,463,202 issued Oct. 31, 1995 to M. Kurosawa et al.
FIG. 1 illustrates the configuration of the tip portion 2 of a gas assist nozzle representative of prior art designs. The nozzle has an internal passageway 4 that terminates in an outlet orifice comprising a conical upstream section 6 and a constant diameter throat section 8. The end face of the nozzle tip has a flat relatively wide annular end surface 14. The outer surface 16 is conically tapered. The nozzle tip portion 2 is shown with its end face confronting a sheet-like workpiece 18. A focused laser beam represented generally at 20 is directed normal to the workpiece. It should be noted, by way of example but not limitation, that the workpiece 18 may be one of the sides or facets of an EFG-grown silicon tube that has an octagonal cross-sectional configuration. Although not shown, it is to be understood that the opposite end of passageway 4 terminates in an inlet orifice that is coaxial with outlet orifice sections 6 and 8. An assist gas flow is introduced under pressure into the nozzle via one or more gas inlet ports 22 and is discharged through the discharge orifice 6, 8 to the region of the workpiece that is undergoing laser cutting. Some of the gas discharged from the nozzle passes through the cut formed in the workpiece by the laser beam, while the remainder of the gas is deflected laterally away from the impact area, flowing outwardly from between the nozzle and the workpiece. The reason that the outer surface 16 extends at an acute angle to the end face section 14 is to facilitate dispersion of the assist gas radially and back away from the workpiece. The velocity of the gas stream directed at the material is determined by the gas pressure on the upstream side of the nozzle orifice and the shape of the nozzle discharge orifice. Regardless of the nozzle orifice design or the gas pressure, a localized force is exerted by the assist gas as it impinges on the material being cut by the laser. The force is increased by increasing the flow (increase in effective orifice size) and/or the gas flow velocity. Depending on the amount of flow and the velocity of the assist gas impinging on the sheet material being cut, the localized force exerted by the assist gas on the material being cut can be substantial. In the case of a thin and fragile material, this force can lead to undesired material fracture. This troublesome situation exists in the case of laser cutting thin EFG-grown silicon tubes to produce wafers for use in making photovoltaic solar cells.
The EFG method of growing crystalline materials from a melt is used to grow doped silicon tubes of selected cross-sectional shapes, e.g., tubes having an octagonal or cylindrical cross-sectional shape. Those tubes are cut by a laser into rectangular wafers, and those wafers are then subjected to various processing steps to convert them into photovoltaic cells. The growth of silicon tubes by the EFG method and the method of processing of silicon wafers to produce photovoltaic cells are well known, as illustrated by the disclosures of U.S. Pat. No. 4,937,053, issued Jun. 26, 1990 to D. S. Harvey, U.S. Pat. No. 4,751,191, issued Jun. 14, 1988 to R. C. Gonsiorawski et al., U.S. Pat. No. 5,698,451, issued Dec. 16, 1997 to J. I. Hanoka, U.S. Pat. No. 5,106,763, issued Apr. 21, 1992 to B. R. Bathey et al. U.S. Pat. No. 5,037,622, issued Aug. 6, 1991 to D. S. Harvey et al., U.S. Pat. No. 5,270,248, issued Dec. 14, 1993 to M. D. Rosenblum et al., and U.S. Pat. No. Re. 34,375, issued Sep. 14, 1993 to B. H. Mackintosh.
It is desirable for a variety of reasons for the EFG-grown silicon tubes to have a wall thickness of 15 mils (0.015 inch) or less, preferably about 6-7 mils. Although other lasers may be used, a Nd:YAG laser has been preferred for cutting EFG-grown silicon tubes into rectangular wafers. The laser beam forms a small circular image or a narrow elongated image at its focal plane. To optimize the cutting speed, it is necessary to focus the laser beam on the surface of the silicon tube being cut. Unfortunately the surfaces of such tubes are not exactly flat, but tend to be uneven as a consequence of undulations and/or random peaks and depressions, with a flatness deviation commonly ranging from about 3 mils to about 7 mils. As a consequence, it is necessary to employ an autofocusing system for the laser so as to keep the laser beam focused on the silicon tube surface as the laser beam is moved relative to the tube.
In addition to having uneven surfaces, silicon tubes having a thickness of 15 mils or less are quite fragile. Although nozzles as shown in FIG. 1 confine gas flow to the immediate area of laser beam impingement (an essential requirement of any effort to limit the forces exerted on the silicon tubes by the assist gas), it is still necessary to have rapid and adequate assist gas flow in order to remove molten material from the kerf. However, achieving a rapid gas flow requires an adequate gas supply pressure, and while an increase in gas pressure will increase the quantity of gas that is discharged by the nozzle, it also increases the force exerted by the assist gas on the material being cut. As a consequence, the laser cutting of thin silicon tubes has been characterized by a frequent fracturing of the wafers being cut out of the tubes. For this reason, there has been a need to limit the localized force exerted on the silicon tubes by an assist gas during the laser cutting operation, while maintaining a high enough gas velocity to efficiently eject molten material from the kerf produced by the laser.
The primary object of this invention is to provide an improved method and apparatus for conducting gas assisted laser cutting of thin and fragile materials.
Another object is to provide a method of laser cutting a fragile material using a gas to remove molten material and laser-generated debris which is characterized by a high velocity gas glow that exerts substantially zero net force on the material being cut.
A further object is to provide a gas assist nozzle for laser cutting which is shaped so as to permit the assist gas to flow at a sonic or supersonic velocity, whereby removal of molten material from the cut region can be accomplished efficiently while the gas flow exerts substantially zero net force on the material being cut.
A further object is to provide a gas assist nozzle for a laser that is characterized by a tip that makes it possible to achieve a zero net force at sufficiently large gaps of 1.0 mm or greater between the nozzle and the material being cut (i.e., different stand-off distances).
The foregoing objects, and other objects rendered obvious by the following description, are achieved by a gas assist nozzle that discharges a high velocity assist gas and is positioned so that the flowing assist gas exerts substantially no net force on the material being cut. In a preferred form, the nozzle is characterized by a tip having a negative conical (concave) end surface and is designed to function as supersonic gas flow device. Other features and advantages of the invention are disclosed or rendered apparent by the following detailed description of the invention which is to be considered together with the accompanying drawings.